London jeweller and designer Glenn Spiro talks about his approach to work

They may not know his name, but jewellery collectors have almost certainly coveted his work.

The world’s most recognisable brands call in Glenn Spiro when they need one-off couture items – which they then present to clients under their own names.

The master jeweller and former Christie’s expert is a designer – about 200 pieces a year leave his Geneva workshop – and one of a handful of dealers worldwide who trade in the rarest and most precious gems, flying them around the globe until they find the right buyer.

“We work with maybe half a dozen super-brands,” says the 51-year-old Londoner, though he does not disclose which ones. “And we buy coloured gems and diamonds – from $10,000 to $10m. Then we design a few pieces for our private collection.”

This is a quiet and insular world, away from the dazzle of Bond Street boutiques. Mr Spiro’s private clients, of which he says there are about 50, typically visit his salon in a smart but anonymous Georgian townhouse in London’s Mayfair. They make their way via a Parisian-style open lift, past a battery of assistants and a vast waiting chamber to his office.

Here they sit opposite Mr Spiro at a generous desk, surrounded by clutter, mirrors and light. To one side, stacked jewellery trays lie across three large open vaults, threatening to tumble and spill their contents across the carpet.

“It’s very private,” he says. “There’s no website, no advertising, no one knows where we are or who we are.” He professes bafflement that clients arrive at all: “People somehow find their way. I don’t know how they turn up, but they do.”

Clients, he explains, typically spend more than $500,000 a year on fine jewellery. So how would a potential client with a big budget find him? “Very complicated. They have to do a bit of work – not that I’m a big shot.”

This exclusive approach is similar to JAR (Joel Arthur Rosenthal), the Paris-based jeweller who works in a hidden atelier off the Place Vendôme. JAR, too, is known for extraordinary gems and reticence.

Mr Spiro likes the comparison: “It’s that type of business, a very private house for a private few customers – that’s who we take care of.”

His somewhat irreverent approach – a disregard for the conventional wisdom on building a business, such as attention to marketing and branding – seems to have served him well. He has operated from his Mayfair office for 11 years.

But it appears to be at odds with his latest move. From April 10, Mr Spiro’s collection will be on sale in Harrods, the London department store. The G London collection will be found in the fine jewellery room, and Mr Spiro will meet international clients in a private Harrods salon, away from the shop floor.

Helen David, fashion director of fine jewellery at Harrods, says customers are increasingly seeking one-off pieces – and she believes Mr Spiro’s “bold, show-stopping high jewellery . . . resonates strongly”.

Mr Spiro says the reason for the move is that he has accumulated a lot of stock: “Why am I doing it? It’s ridiculous that it’s all stuck in a safe – that’s why.” But he eventually adds that he would like new clients: “Ten new customers, or 20 – I’ll be over the moon with 20.”

With the same irreverence, he claims to have no business case for the Harrods move: “I’ve done zero research, I’m completely winging it. But I’d be surprised if there are more than 2,000 people in the world spending more than half a million dollars a year on jewellery.”

The G London collection is priced accordingly, with pieces starting at $100,000 and ranging to about $25m.

The designs are riotous and exuberant, but it is hard to point to a signature style. Some pieces, such as a pair of chandelier earrings made in 18-carat pink and white gold and set with rubies and diamonds, take modernist lines. Others such as a tassel pendant set with natural pink conch pearl and a 11.16 carat cognac diamond might appeal to more bohemian tastes.

Trends, he says, hold no interest and neither does he create jewellery with a particular client in mind.

“There is no uniform, it’s just cool, out-there,” he says. “I don’t know how I’m going to feel. Anyway, how do I know who’s coming? I have no idea, so I just make what I like.”

Some clients, he says, are sent away. “I know a customer, who wants to buy something for a lot of money. And I know exactly where it is, and I can sell it, but I won’t do it. She’s very upset with me, because I don’t like the stone she wants to buy and I’m not going to buy it. I don’t like the diamond – it’s horrible, it’s a modern stone that I don’t really like.”

What is it? “It’s a princess cut. They have no charm, they are manufactured by a computer.”

But finding the calibre of stones that meet his standards, from auctions, other dealers and private collections is, he says, increasingly difficult. “There are more wealthy people in the world [than previously] and they’ve bought most [high-calibre stones] – annoyingly.

“You always had to fight to get the best stones, and it was always a small business, so when those stones sold, [producers] dug a bit deeper and they produced a bit more – and now there are very average stones.

“There is too much,” he adds. “High-end jewellery has become a mass business.”

It is clear why clients visit: he is lively and good company. When I admire his work, he insists I try on piece after piece, piling them high on the desk, from a titanium and diamond cuff set centrally with a 40.15-carat, certified D, internally flawless type 11a diamond the size of a small mirror, to a twistable white gold and diamond pavé ring that opens, flower-fashion, to reveal a 1.01 carat pink diamond.

Will the Harrods move detract from the mystique, the effort that it takes for customers to find him? And does he risk becoming associated with the mass business that troubles him?

He hopes not: “I’m not looking for change. But I need business, so this is my route to maybe finding a few more people to buy more things.”

The CV

● Cartier: English artworks, 1977-83

● Robert Glenn: Chairman, 1983-91

● Christie’s: Senior international director and jewellery specialist, 1992-2000

● G London: Founder and chairman, 2000